This study will use electrophysiology to determine whether age-related differences in ADHD symptom expression are related to differences in brain activity involved with response inhibition. Furthermore, by quantifying ADHD brain function both on and off psychostimulant medication, it will be possible to determine how age influences ADHD persons' neural response to methylphenidate. We broadly hypothesize that methylphenidate will affect each groups' ERPs differently. Medication-related differences also will be observed in ADHD symptom severity and neuropsychological test performance. Innovations include that this will be the first electrophysiology study 1) of response inhibition in ADHD adults, 2) that attempts to directly link brain function changes to symptomatology changes in maturation, and 3) that examines medication effects on cognition and behavior across age groups in ADHD.